1. Field of the Invention
The present invention relates to a liquid crystal composition and, more particularly, to a liquid crystal composition which can be suitably adapted in a liquid crystal device controlled by a two frequency addressing scheme.
1. Description of the Prior Art
More liquid crystal devices are recently being used in television sets, computer terminal devices and OA equipment. In many such liquid crystal devices, a number of pixels are arranged in a matrix form and are time-divisionally driven. In a liquid crystal device of this type, when the number of pixels increases, the number of scanning lines increases and the time-division number also increases. For this reason, the contrast between ON/OFF pixels cannot be kept high enough due to the cross effect phenomenon or the like. In addition, liquid crystal devices of this type have only a slow response speed when assembled in a computer terminal device or OA equipment.
In view of this problem, the two frequency addressing scheme utilizing the dielectric dispersion phenomenon of a liquid crystal has been proposed. In this scheme, a signal voltage of a high frequency (e.g., 100 kHz) is applied to a liquid crystal so as to align the liquid crystal molecular axis perpendicular to an electric field, and a signal voltage of a low frequency (e.g., 200 Hz) is applied so as to align the liquid crystal molecular axis parallel to the electric field. In accordance with this scheme, the time-division number can be increased, and quick response can be obtained.
As a liquid crystal for use in a liquid crystal device driven by the two frequency addressing scheme, Japanese Patent Disclosure No. 57-83577 discloses a liquid crystal composition consisting of various liquid crystal compounds.
However, when conventional liquid crystal compositions, including the one disclosed in the above-mentioned Disclosure, are driven by the two frequency addressing scheme, the response characteristic upon application of an electric field of low frequency is degraded if an electric field of high frequency has been applied to the liquid crystal for a considerable period of time. As described above, when high- and low-frequency electric fields are applied, liquid crystal molecules are aligned parallel and perpendicular to the substrate. However, it should be noted that liquid crystal molecules are not completely aligned parallel or perpendicular to the substrate but are slightly inclined with respect thereto.
When a high-frequency electric field is applied for an extended period of time, liquid crystal molecules are aligned almost parallel to the substrate. When a low-frequency electric field is subsequently applied, liquid crystal molecules cannot immediately be aligned vertically (high-frequency hysteresis effect). When a positive display TN-type liquid crystal device is driven by the two frequency addressing scheme, the operation for closing the shutter is delayed. When a guest-host type liquid crystal device is driven by the two frequency addressing scheme, the operation for opening the shutter is delayed.
Conventional liquid crystal compositions have a response time of, at best, 1 msec. As such, conventional liquid crystal compositions cannot be used to obtain high-speed computer terminal devices or OA equipment.
Moreover, conventional two frequency addressing liquid crystal compositions have a high drive voltage (e.g., 30 V) and a small tolerance for high breakdown voltages for semiconductor ICs constituting drivers.